1. Introduction: Pugs & Cardiac Assessment
Pugs occupy a unique and often challenging space in veterinary cardiology and diagnostic imaging. As a brachycephalic breed with extreme thoracic and airway conformation, they frequently “break the rules” that apply to mesocephalic and dolichocephalic dogs. Nowhere is this more apparent than in cardiac assessment, particularly when using radiographic reference values such as Vertebral Heart Score (VHS) and Vertebral Left Atrial Size (VLAS).
In general practice, thoracic radiographs remain one of the most common first-line tools for evaluating heart size. However, applying generic reference ranges to pugs can lead to significant overdiagnosis of cardiomegaly, unnecessary anxiety for owners, and potentially inappropriate treatment decisions. A pug that appears to have a markedly enlarged heart by traditional standards may, in fact, be completely normal for the breed.
The core issue lies in anatomy. Pugs have a compact, wide, barrel-shaped thorax with shortened ribs, altered vertebral proportions, and a heart that naturally appears rounder and more globoid on lateral and dorsoventral views. These features directly influence objective measurements such as VHS and VLAS, pushing them well above commonly taught “normal” cutoffs.
Complicating matters further is the high prevalence of Brachycephalic Obstructive Airway Syndrome (BOAS). BOAS does not only affect respiration; it has well-documented cardiovascular consequences, including pulmonary hypertension and secondary right-sided cardiac changes. Clinical signs such as exercise intolerance, cyanosis, and respiratory distress may stem from cardiac disease, respiratory disease, or—most commonly—both.
For these reasons, breed-specific reference values are not a luxury in pugs; they are a necessity. Understanding what constitutes normal VHS and VLAS in this breed is essential for accurate diagnosis, appropriate staging of conditions like myxomatous mitral valve disease (MMVD), and informed decision-making regarding echocardiography and treatment.
This article provides an in-depth, evidence-based guide to interpreting VHS and VLAS in pugs, with special emphasis on the interaction between brachycephalic conformation, BOAS, and cardiac disease.
2. Pug VHS Reference Values
Normal VHS for Pugs
Multiple studies evaluating breed-specific VHS values have demonstrated that pugs have some of the highest normal VHS measurements among small-breed dogs. The currently accepted reference value for pugs is:
- Mean VHS: 11.25 ± 0.62 vertebrae
- 95% reference range: 10.1–12.8 vertebrae
These values are notably higher than the traditionally cited general canine reference of approximately 10.5 vertebrae. Published studies, including Jepsen-Grant et al. (2013), have documented elevated breed-specific VHS ranges in brachycephalic breeds, confirming that standard cutoffs are not appropriate for these populations.
Why General Cutoffs Fail in Pugs
In many small breeds, a VHS of 11.5 vertebrae is commonly used as a red flag for cardiomegaly and, in the context of MMVD, may trigger classification as ACVIM Stage B2. However, this cutoff should be applied with caution in pugs.
Critical point: An elevated VHS can be normal for brachycephalic breeds like pugs, but should still be evaluated in clinical context. A VHS of 11.5 vertebrae falls within the breed-adjusted reference range for pugs and does not, by itself, indicate cardiomegaly.
Applying non–breed-adjusted criteria can result in:
- Overdiagnosis of cardiomegaly
- Premature initiation of cardiac medications
- Unnecessary echocardiography
- Misclassification of MMVD stage
Why Do Pugs Have Such High VHS Values?
Several anatomic and conformational factors contribute to the elevated VHS in pugs:
1. Extreme Thoracic Conformation
Pugs have a markedly shortened thorax with reduced cranial–caudal length. When the heart occupies a shorter thoracic cavity, it naturally spans more vertebrae on lateral radiographs.
2. Wide, Barrel-Shaped Chest
The increased thoracic width results in a heart that appears broader and more rounded, particularly on dorsoventral or ventrodorsal views.
3. Compact Thorax
The compact nature of the pug thorax exaggerates cardiac silhouette size relative to vertebral length, artificially inflating VHS measurements.
4. Globoid Cardiac Silhouette
Even in healthy pugs, the heart often appears rounder and less elongated than in other breeds, contributing to increased long-axis and short-axis measurements.
Breed-Adjusted Cutoffs for Cardiomegaly
Rather than relying on generic VHS thresholds, clinicians should apply breed-adjusted interpretation:
- Within breed-adjusted reference range: VHS ≤12.5 (can be normal for pugs, but should be interpreted in clinical context)
- Borderline / interpret with caution: VHS 12.5–13.0
- Suspicious for true cardiomegaly: VHS >13.0
Important: No single VHS value should be considered "always normal" for any breed. Breed-adjusted reference ranges indicate that elevated values can be normal, but clinical context—including auscultation findings, clinical signs, serial trends, and VLAS—must always inform interpretation. Published breed-specific data (e.g., Jepsen-Grant et al., 2013; Bavegems et al., 2005) support the use of higher reference ranges for brachycephalic breeds but do not eliminate the need for individualized assessment.
Importantly, VHS should never be interpreted in isolation. Clinical signs, auscultation findings, VLAS, and—when indicated—echocardiography must all be integrated into the assessment.
3. Pug VLAS Reference Values
Understanding VLAS in Pugs
Vertebral Left Atrial Size (VLAS) has become an increasingly popular radiographic metric for assessing left atrial enlargement, particularly in dogs with MMVD. In pugs, however, VLAS interpretation requires the same breed-specific caution as VHS.
While large-scale pug-specific VLAS datasets are more limited than VHS studies, available evidence and clinical experience suggest that pugs tend to have mildly increased baseline VLAS values compared to non-brachycephalic breeds.
Normal VLAS Trends in Pugs
- Mildly higher VLAS values can be normal
- Slight left atrial prominence may be present without true pathology
- Borderline VLAS enlargement does not automatically equate to MMVD progression
Relationship to BOAS Severity
Emerging research indicates a correlation between BOAS severity and increased VLAS in brachycephalic dogs, including pugs. Chronic upper airway obstruction can lead to:
- Increased negative intrathoracic pressure
- Altered venous return
- Secondary pulmonary hypertension
- Right-sided cardiac changes that can indirectly affect left atrial appearance
In dogs with severe BOAS, VLAS may be increased even in the absence of primary left-sided cardiac disease.
Interpretation Considerations
When interpreting VLAS in pugs:
- Always assess airway status
- Correlate with respiratory noise and effort
- Avoid overinterpreting mild VLAS increases
- Confirm suspected enlargement with echocardiography
VLAS is a useful tool, but in pugs it should be viewed as a screening parameter, not a definitive diagnostic endpoint.
4. The Brachycephalic Challenge
BOAS and Cardiac Effects
Brachycephalic Obstructive Airway Syndrome is not merely a respiratory disorder—it is a multisystem disease with profound cardiovascular implications. Chronic upper airway obstruction leads to persistent increases in inspiratory effort and negative intrathoracic pressure, placing abnormal stress on the heart and pulmonary vasculature.
Pulmonary Hypertension in Brachycephalics
Pugs are predisposed to pulmonary hypertension due to:
- Chronic hypoxemia
- Pulmonary vascular remodeling
- Increased pulmonary arterial pressure
Pulmonary hypertension can cause:
- Right ventricular hypertrophy
- Right atrial enlargement
- Tricuspid regurgitation
These changes can alter overall cardiac silhouette and complicate radiographic interpretation.
Effects of Chronic Hypoxemia
Long-term hypoxemia associated with BOAS can result in:
- Polycythemia
- Increased blood viscosity
- Increased cardiac workload
Over time, this may contribute to both functional and structural cardiac changes.
Differentiating Cardiac from Respiratory Signs
Many clinical signs overlap:
- Stertor/stridor: Primarily upper airway
- Exercise intolerance: Cardiac or respiratory
- Cyanosis: Cardiac or respiratory
- Syncope: Either, or both
Because of this overlap, radiographic findings must be interpreted alongside airway evaluation.
When to Evaluate the Heart in a Pug with Respiratory Signs
Cardiac evaluation is warranted when:
- Murmurs are present
- Exercise intolerance is disproportionate to airway findings
- Syncope or collapse occurs
- Radiographs show changes beyond breed-expected norms
5. Common Cardiac Conditions in Pugs
Myxomatous Mitral Valve Disease (MMVD)
MMVD is common in pugs, though often with later onset compared to Cavalier King Charles Spaniels. Accurate staging is critical, and breed-adjusted VHS and VLAS interpretation is essential to avoid misclassification.
Pulmonic Stenosis
Pulmonic stenosis is relatively common in pugs and may contribute to right-sided cardiac enlargement and exercise intolerance.
Tricuspid Valve Dysplasia
Congenital tricuspid valve abnormalities can lead to right atrial and ventricular enlargement, complicating radiographic assessment.
Pulmonary Hypertension Secondary to BOAS
This is a frequent and underrecognized problem in pugs. Radiographs may show enlarged pulmonary arteries and right-sided cardiomegaly.
Arrhythmias
Both supraventricular and ventricular arrhythmias may occur, particularly in dogs with advanced disease or hypoxemia.
6. Radiographic Interpretation in Pugs
Unique Thoracic Conformation
Pugs have:
- Short ribs
- Wide thoraces
- Altered vertebral proportions
These features exaggerate cardiac size on radiographs.
Heart Position Differences
The heart often sits more cranially and appears more vertical, affecting long-axis measurements.
What “Normal” Looks Like
A normal pug heart may appear:
- Large
- Round
- Broad-based
This is not automatically pathological.
Common Misinterpretations
- Overcalling cardiomegaly
- Misidentifying normal bulges as chamber enlargement
- Ignoring breed norms
When to Trust VHS vs Subjective Assessment
Objective measurements are useful, but in pugs, context is everything. VHS should be weighed against clinical and echocardiographic findings.
7. BOAS-Cardiac Connection
Research Insights
Studies demonstrate measurable differences in cardiac dimensions between brachycephalic dogs with mild versus severe BOAS.
How BOAS Affects Cardiac Values
- Increased VHS and VLAS
- Right-sided changes
- Pulmonary artery enlargement
VHS/VLAS Correlation with BOAS Severity
More severe BOAS is associated with higher radiographic cardiac measurements, even without primary heart disease.
Integrated Assessment Approach
Optimal assessment includes:
- Airway evaluation
- Radiography
- Echocardiography
- Clinical signs
8. Clinical Decision Making
When VHS Suggests True Cardiomegaly
In pugs, true cardiomegaly is more likely when:
- VHS >13.0
- Progressive increase over time
- Concurrent VLAS enlargement
- Clinical signs support cardiac disease
Echocardiography Indications
- Murmurs ≥ grade III/VI
- Syncope
- Suspected pulmonary hypertension
- Ambiguous radiographic findings
Balancing Respiratory and Cardiac Concerns
Treatment plans should address both airway and cardiac disease when present, rather than focusing on one in isolation.
9. Case Example
Pug with VHS 12.2 – Is This Normal?
A 7-year-old pug presents for evaluation of exercise intolerance. Thoracic radiographs reveal a VHS of 12.2 vertebrae. There is mild bronchial patterning but no overt pulmonary edema. Auscultation reveals a soft grade II/VI left apical systolic murmur.
Clinical reasoning:
- VHS of 12.2 falls within the normal pug reference range
- No strong radiographic evidence of left atrial enlargement
- Exercise intolerance may be related to BOAS
Next steps:
- Airway assessment
- Consider echocardiography to characterize murmur
- Avoid premature cardiac medication
10. Summary & Key Points
Pugs require a fundamentally different approach to radiographic cardiac assessment. Their breed-adjusted VHS and VLAS reference ranges are higher than generic canine values, and failure to account for this leads to frequent misdiagnosis. BOAS further complicates interpretation by influencing both clinical signs and cardiac measurements. Accurate assessment depends on breed-specific reference values, careful clinical correlation, and judicious use of echocardiography. When evaluating a pug, always remember: elevated VHS values can be normal for this breed, but must still be evaluated in the full clinical context.
References
- Jepsen-Grant K, Pollard RE, Johnson LR. Vertebral heart scores in eight dog breeds. Vet Radiol Ultrasound. 2013;54(1):3–8.
- Bavegems V, Van Caelenberg A, Duchateau L, et al. Vertebral heart size ranges specific for whippets. Vet Radiol Ultrasound. 2005;46(5):400–403.
- Buchanan JW, Bücheler J. Vertebral scale system to measure canine heart size in radiographs. J Am Vet Med Assoc. 1995;206(2):194–199.
Related Resources
- Breed-Specific VHS Comparison Chart - Compare VHS and VLAS values across multiple breeds
- VHS (Vertebral Heart Score) Complete Guide - Learn the fundamentals of VHS measurement
- VLAS (Vertebral Left Atrial Size) Guide - Understanding left atrial measurements